High-Efficiency Interdigitated Electrode-Based Droplet Merger for Enabling Error-Free Droplet Microfluidic Systems

Merging two droplets into a droplet to add and mix two contents is one of the common droplet microfluidic functions with droplet generation and sorting, performing broad ranges of biological and chemical assays in droplets. However, traditional droplet-merging techniques often encounter unsynchronized droplets, causing overmerging or mis-merging, and unwanted merging outside of the desired zone. This is more severe when the incoming droplets to be merged are polydisperse in their sizes, often observed in assays that require long-term incubation, elevated-temperature, and/or multiple droplet processing steps. Here, we developed an interdigitated electrode (IDE)-based droplet merger consisting of a droplet autosynchronizing channel and a merging channel. The autosynchronizing channel provides >95% merging efficiency even when 20% polydispersity in the droplet size exists. The highly localized and enhanced dielectrophoretic force generated by the IDEs on the channel bottom allows droplet merging at an extremely low voltage (4.5 V) and only locally at the IDE region. A systematic evaluation of how various design and operation parameters of the IDE merger, such as IDE finger dimensions, dielectric coating layer thickness, droplet size, and droplet flow speed impact the performance was conducted. The optimized device showed consistent performance even when operating for up to 100 h consecutively at high throughput (100 droplets/s). The presented technology has been integrated into a droplet microfluidics workflow to test the lytic activities of bacteriophage on bacterial host cells with 100% merging efficiency. We expect this function to be integrated into droplet microfluidic systems performing broad ranges of high-throughput chemical and biological assays.


Figures S1 to S7
Legends for movies S1 to S6 Other Supplementary material for this manuscript includes the following:

SUPPLEMENTARY TEXT Device fabrication details
The borosilicate glass substrate was first cleaned using Piranha solution (3:1 ratio of sulfuric acid and hydrogen peroxide), followed by electron-beam evaporation (PVD 75 Electron Beam Evaporation Tool, Kurt J. Lesker Company ® , PA, USA) to create a Ti and Au layer.The thicknesses of Ti and Au are 20 nm and 100 nm, respectively.Photolithography using a photoresist (AZ-5214, MicroChem ® , Westborough, MA, USA) was then conducted by spin-coating the photoresist (1 st spinning: 500 rpm, 10 sec; 2 nd spinning: 4,000 rpm, 30 sec), then prebaking the substrate on a hot plate for 1 min (65 °C) and then for 3 min (95 °C).Lithography was conducted using an EVG 610 Double-sided Mask Aligner (separation: 10 µm, mode: hard contact, power: 135 mJ), developed by AZ726 MIF developer, and then the Ti layer etched using hydrofluoric acid (HF) and the Au layer etched using Au etchant (Gold Etchant, standard, Sigma-Aldrich, Saint Louis, MO, USA), followed by rinsing with acetone to remove the photoresist 1, 2 .
The microfluidic device was fabricated in polydimethylsiloxane (PDMS, Sylgard 184 Dow Corning, MI, USA) using a conventional soft lithography process.To fabricate the master mold, a photoresist (SU-8 ® 2050, MicroChem, Westborough, MA, USA) was spin-coated (10 sec at 500 rpm, 30 sec at 4,000 rpm), soft baked on a hot plate (95⁰C, 10 min), patterned using an EVG 610 Double-sided Mask Aligner (separation: 10 µm, mode: soft contact, power: 220 mJ), post-exposure baked (60⁰C, 2 min, then 95⁰C, 10 min), and developed by MIF-319.Then, polydimethyl siloxane precursor (PDMS, Sylgard 184, Dow Corning, MI, USA) was mixed with curing agent at a 10:1 ratio, degassed, poured on the master mold in a petri dish to form a height of 15 mm and a diameter of 100 mm, baked in an oven (70⁰C) for 24 h, and peeled off from the petri dish.The baked PDMS block was punched to form microfluidic diameter of 0.635 mm inlets and outlet.Plasma treatment using an oxygen plasma (Harrick Plasma PDC-001-HP, 18 W for 120 s) was conducted to bond the glass layer with metal to the PDMS microfluidic layer.

Calculation of droplet size using a Matlab ® script
In order to calculate the size of droplets, droplet imaging is required.The provided functions, "regionprops", "cat", "imfindcircles", and "viscircles", were employed to find and measure the droplet size.Once droplet sizes were obtained, then the probability density function (PDF) of the size distribution was plotted through an inherited function, "pdf".
Illustration of the microfabrication steps.ESI Figure S2.Basic working principle, advantages, and disadvantages of (A) IDE-based droplet merger and (B) conventional droplet merger.A: The IDE droplet merger generates a highly localized electric field to merge paired droplets, eliminating the need for 3D electrode shielding to isolate the electric field, even during high-throughput droplet merging.B: A conventional droplet merger creates a strong and extensive electric field to merge paired droplets, often leading to unwanted merging outside the desired merging zone and over-merging.ESI Figure S3.A COMSOL Multiphysics simulation result showing electric field generated by the IDE structure.A: Top view of the electric field showing highly localized electric field that does not affect the droplets outside of the IDE regions.B: Side view of the electric field showing that the electric field reaches up to 20-30 µm from the IDE patterns, can physically intact droplets flowing on the IDE structure to merge them.ESI Figure S4.Probability density function (PDF) of the resulting droplet sizes before and after the 12 h incubation step.Average sizes of droplets before and after incubation were 71.6 ± 7.2 µm and 73.2 ± 14.2 µm, respectively.ESI Figure S5.Design of the conventional 3D liquid metal electrode-based droplet merging device used for comparison to the IDE-based droplet merging device.ESI Figure S6.Merging of paired droplets under small droplet-to-droplet spacing using the IDE merger.A: The IDE droplet merging device performs under extremely small droplet pair-to-pair distance (15 µm shown in this figure) without affecting the neighboring droplet pairs due to its highly localized electric field for merging droplets.B: The conventional droplet merger device having an 3D electrode requires a larger droplet pair-to-pair distance to avoid over-merging due to its strong and wide electric field.Images within the red box shows an example of an unwanted over-merged droplet outside of the 3D metal electrode.Scale bar = 100 µm.ESI Figure S7.Image of a damaged IDE without Si 3 N 4 coating after 4 h of continuous droplet merging operation.Scale bar = 100 µm.